Open tubular columns (OTCs) have been in use in chromatography for several decades. They typically consist of an inert tube with a stationary phase deposited on the inner walls with a constant cross-sectional area along the length of the tube. The stationary phase provides a partitioning phase that permits chemical separation of bands of analyte molecules carried in the mobile phase passed therethrough. The dimensions of the tube, thickness of the stationary phase, and composition of the stationary phase are chosen by the chromatographer to produce the desired separation.
To form OTCs, two methods are generally used to deposit stationary phase inside the open tube; dynamic and static stationary phase deposition. In a dynamic coating process, a plug of solvent, containing dissolved materials that will become the stationary phase, is flowed through the tube such that the dissolved material is left on the inner wall of the tube forming the initial form of the stationary phase film or coating. In some cases, additional treatment steps are implemented, such as heating and drying. These steps seek to condition or chemically alter the residual stationary phase film or active sites in the tube until the desired characteristics are provided to the stationary phase. Dynamic coating is often not preferred because it is difficult to control and therefore less economic for column production.
In contrast, static coating allows direct, relatively accurate, estimation of the deposited film thickness and can also produce a more uniform film. In static coating methods, one or more materials that will become the stationary phase are suspended or dissolved in a liquid that is used to fill the tube that will become the OTC. One end of this tube is sealed off and the other connected to a vacuum source. In some approaches, a temperature bath may be used to maintain the temperature of the filled tube. The liquid is then allowed to evaporate, which may be include the use of a low-pressure vacuum source. During evaporation, the meniscus (air/liquid interface inside the tube) retreats and leaves behind a residual film of the material(s) that will become the stationary phase. The residual film may then be further chemically treated, conditioned, etc to form the final stationary phase.
Variable Geometry Columns (VGC) are disclosed in U.S. Patent Publication No. 2009/0139934 to Steinecker et al., herein incorporated by reference in its entirety. That patent application discusses chromatography columns that produce differential acceleration based separations by several mechanisms. Some proposed mechanisms include increasing the cross-section of the column toward the column outlet, changing the thickness or other composition of stationary phase within the column, and providing a temperature and/or mobile phase velocity gradient along the column. Methods and apparatus for efficiently and cost-effectively making VGCs are disclosed herein.